1. Field of the Invention
The present invention relates to wireless networks. More particularly it relates to multi-channel Media Access Control (MAC) operating on a multiple radio or Wireless Local Area Network (WLAN) device.
2. Description of Related Art
The WiMedia MAC and Physical layer (PHY) specification (ECMA, “Standard ECMA-368: High Rate Ultra Wideband PHY and MAC Standard,” 2005, incorporated herein by reference) has originated from the Multiband OFDM Alliance (MBOA) proposal.
The goal of a future WiMedia system will be to provide a usable MAC layer throughput in excess of 1 Gbps. For example, a future WiMedia PHY may achieve higher rates by bonding two channels together, providing 2 spatial streams with Multiple Input-Multiple Output (MIMO) and employing a higher rate channel code.
The WiMedia PHY transmits data in units of six Orthogonal Frequency Division Multiplexing (OFDM) symbols instead of the single OFDM symbol unit of 802.11a. The following equations can be used to calculate the characteristics of the highest rate future WiMedia PHY modes using the parameter values shown below in Table 1.
TABLE 1Future maximum rate PHY mode parametersNIBP6S = ND · Nbits—carrier  · Nss · R   Rate  =            N              IBP        ⁢                                  ⁢        6        ⁢                                  ⁢        S                            N                  interleaved          ⁢          _          ⁢          sym                    ·              T        sym             ParameterValueND (Number of data sub-carriers)Single channel100Bonded21channel10Nbits—carrier (The number of bits per2subcarrier2)Nss (Number of spatial streams)2R (Coding Rate)7/8Tsym (OFDM symbol period)312.5nsNinterleaved—sym (Number symbols for6interleaving)Notes:1Assuming that the 2 guard carriers at the centre of the individual channels and the 9 guard carriers between the individual channels are gained and that 1 carrier is lost to provide a guard carrier at the centre of the bonded channel.2Assuming DCM modulation (QPSK will also provide 2 bits per sub-carrier)
The characteristics of potential future WiMedia PHY modes are shown in Table 2.
TABLE 2Future maximum rate PHY mode (single and bonded channels)NIBP6SRate(Info bits/6 OFDM(Mbps)symbols)Single Channel11202100Bonded23524410channel
The introduction of MIMO to provide two spatial streams will result in an increase in the size of the physical layer convergence protocol (PLCP) preamble overhead. If the approach taken in the greenfield preamble of 802.11n is replicated, it can be assumed that a two spatial stream MIMO preamble for the WiMedia PHY will consist of the same synchronisation sequence but will now have two channel estimation sequences. This will result in the standard preamble increasing from 9.375 μs to 11.25 μs and the burst preamble increasing from 5.625 μs to 7.5 μs.
The PHY assumptions derived above can be used to determine the MAC layer throughputs that can be expected. The Wimedia MAC provides two main access methods. The first is Prioritised Channel Access (PCA), which is almost identical to 802.11's Enhanced Distributed Coordination Function (DCF) Channel Access (EDCA); the only differences being in the parameter values. The second access method is the Dynamic Reservation Protocol (DRP) which allocates timeslots to a particular user. Unacknowledged bursts may be described as a third access method; they employ the Minimum Inter Frame Space (MIFS) and the burst preamble and operate over a point-to-point link. The MIFS is shorter than the Short Inter Frame Space (SIFS) because it does not need to include the radio turnaround time. The burst preamble is shorter than the standard preamble because the synchronisation sequence can be reduced due to the retained knowledge that the receiver gained from the previous frame.
FIG. 1 shows two sets of results for the three access methods described above. Beacon overheads have been ignored for the sake of illustrating a theoretical maximum throughput. The first set of results is for a 1500 byte MAC Service Data Unit (MSDU). This is historically the original maximum Ethernet frame size. The maximum User Datagram Protocol (UDP) frame is typically 1024 bytes and the maximum Transmission Control Protocol (TCP) frame is 64 kbytes. TCP frames are segmented into Ethernet frames and then put into 802.11 frames. The second set of results is for a 4095 byte MSDU, which is the maximum MSDU size supported by the WiMedia MAC. If TCP or UDP protocols are employed then a 4095 byte MSDU can only be achieved by frame aggregation.
FIG. 1 illustrates that even with a 2.35 Gbps PHY a 1 Gbps MAC throughput (approximately 50% of the available PHY rate) can only be achieved with a single point-to-point link, without MAC overheads (control and management frames) and employing a frame size that is larger than what the upper layers may generate. It is therefore doubtful if a reliable service can be provided without control and management frames once the streaming has begun.
Getting such low medium capacity utilization is mainly a consequence of the need to spend time for carrier sensing, signal propagation (i.e. IFS) and channel estimation (e.g. PLCP preamble) in a high data rate PHY medium, regardless whether using bonded channels or not.
An improvement can be gained by increasing the amount of time spent transmitting data by increasing the maximum MSDU size beyond 4095 bytes, or, similarly, introducing/enhancing frame aggregation. This might be a valid technique for some of the application specific applications that will have requirements of 1 Gbps MAC throughputs. Packets of this size may not be as enormous as they first seem, because the probability of errors occurring is a function of the transmission time and not of the packet size directly. A packet of 4095 bytes takes only 15 μs to transmit on a 2.35 Gbps PHY, which is much shorter than the coherence time of the channels that will be encountered.
In: Luca De Nardis, Guerino Giancola, Maria-Gabriella Di Benedetto, “A power-efficient routing metric for UWB wireless mobile networks”, Vehicular Technology Conference 2003, VTC 2003-Fall. 2003, Volume: 5, pages 3105-3109, incorporated herein by reference, a method is discussed for setting up connections by optimizing a power-dependent cost function. Results show that the power-saving strategy leads to multi-hop communication paths between terminals within reach of each other (physical visibility) and by this way increases network performance.
Optimal power control, scheduling, and routing in UWB networks is discussed, for example, in Radunovic, B., Le Boudec, J.-Y., “Optimal power control, scheduling, and routing in UWB networks”, IEEE Journal on Selected Areas in Communications, September 2004, Volume: 22, Issue: 7, pages 1252-1270, incorporated herein by reference. In this certain approach the objective is to maximize flow rates under given node power constraints (there is a linear dependence between rate and signal-to-noise ratio at the receiver). The suggested optimal routing, scheduling and power control solution is characterized by the following assumptions:
1) When data is being sent over a link, it is optimal to have an exclusion region around the destination, in which all nodes remain silent during transmission, whereas nodes outside of this region can transmit in parallel, regardless of the interference they produce at the destination. Additionally, the source adapts its transmission rate according to the level of interference at the destination due to sources outside of the exclusion region.2) The optimal size of this exclusion region depends only on the transmission power of the source of the link, and not on the length of the link or on positions of nodes in its vicinity.3) Each node in a given time slot either sends data at the maximum power, or does not send at all.4) As for the routing, by restricting to a subset of routes where on each successive hop the distance is decreased toward the destination, it is shown that relaying along a minimum energy and loss route is always better than using longer hops or sending directly, which is not obvious since rate is optimised and not power consumption.5) Finally the design of the optimal MAC protocol is independent of the choice of the routing protocol.
For narrowband networks, assumptions 2), 4), and 5) do not hold, which shows that the design of an UWB network should be addressed in a different way than for narrowband.
However, this is just one approach to the multi-hop problem. For example, one could argue that in Non Line of Sight (NLOS) cases that relaying along minimum energy and loss routes may be inefficient as compared with lowering the rate and increasing the range. The MAC may therefore be dependent of routing decisions as we can dynamically decide whether to trade-off data-rate for robustness to multi-path (and improvement of performance in NLOS), which is needed in order to make good routing decisions. Also, in Line of Sight (LOS) case with (for instance) random node deployment it is still not clear whether multi-hop routing will always improve performance.
In order to improve the throughput performance of WLAN MAC schemes some researchers have proposed to split the single shared channel into two subchannels: a control subchannel and a data subchannel. The control subchannel is used for access reservation to the data subchannel over which the data packets are transmitted (J. Deng, Y. S. Han, and Z. J. Haas, Analyzing Split Channel Medium Access Control Schemes with ALOHA Reservation, Proc. Second Int'l Conf. AD-HOC Networks and Wireless, October 2003, incorporated herein by reference).
Generally, multi-channel MACs utilize more than one channel (e.g. a combination of data and signalling ones) in order to organize access in a more efficient manner. Multi-channel MACs usually address the optimisation problem of organising access with 2 or more channels within either single or multiple hops.
In the 802.11s Common Channel Framework (CCF) approach by Sung-Won Lee and Rakesh Taori; “Common Channel Framework: A Simple Multi-Channel MAC Framework for 802.11s Mesh Network”, IST Summit 2006, incorporated herein by reference, the gain in performance is due to different devices using the different channels at the same, but with a control channel used to assign “contention free” access to the other channels.
Multi-rate multi-channel MACs (Niranjan, S. Pandey, and A. Ganz, “Design and evaluation of multichannel multirate wireless networks,” Mobile Networks and Applications, Vol. 11, issue 5, pp. 697-709, 2006, incorporated herein by reference.) refer to the problem of having a given number of channels and trying to figure out how to optimally assign channels to links meeting certain traffic-based criteria (more specifically multiple traffic rates). The typical optimization objective is the reduction of multi-rate interference (i.e. “slow” rates “annoying” “high” rates).
Bonded channels have been considered in the 802.11n draft amendment to the standard by the IEEE task group TGn (IEEE P802.11n™/D1.02, Draft Amendment to STANDARD for Information Technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Enhancements for Higher Throughput, incorporated herein by reference). This document describes a Phased Coexistence Operation (PCO) as an optional coexistence mechanism in which a BSS operates in alternating 20 MHz and 40 MHz phases under the control of a PCO AP. The PCO AP reserves the 20 MHz control channel and the 20 MHz extension channel in turn to start the 40 MHz phase and resets the NAV in the 20 MHz channels in the opposite order to start the 20 MHz phase.
Utilising dual WLAN channels for “double” transmission rates has been commercialised in the Atheros chip (Dynamic Turbo technology). The Atheros chips have been widely used within 802.11g wireless router/gateway products by many manufacturers (including Toshiba, Sony, Netgear, NEC, Fujitsu, Gigabyte, D-Link, etc). With this technology, manufacturers claim to have achieved maximum data rates of 2×54 Mps=108 Mps.
Dynamic Turbo is similar to trunking techniques used in Fast Ethernet networks (which use two or more wires to increase overall bandwidth). Briefly described, Dynamic Turbo is designed to automatically double the realized bandwidth when required by handling two channels as one. Dynamic Turbo is engaged based on network traffic requirements and environmental conditions. Access points switch dynamically to this high-performance mode when an associated wireless station requires greater bandwidth based on the sustained throughput between the link between the access point and the station pair. (Atheros Communications White Paper: Super G: Maximizing Wireless Performance, 2004, incorporated herein by reference).
The use of 2 channels in 802.11 to “double” the data rate comes at the expense of range. This is because the same Effective Isotropic Radiated Power (EIRP) (of 100 mW in Europe) is necessary (governed by regulation) and so we cannot increase power, just spread it across a larger bandwidth. For UWB the rules may be different (based more on power spectral density than EIRP) and this may mean we can gain both data rate and range with the “dual radios” or channel bonding approaches. Obviously at the expense of increased device complexity and cost.
It is an object of the present invention to obviate at least some of the above disadvantages and provide an improved network performance at high data rates.